1. Field of the Invention
This invention relates to paging receivers and more particularly to a paging receiver memory having selectively protected regions of memory.
2. Background of the Invention
Communications systems in general and paging systems in particular using transmitting call signals have attained widespread use for calling selected receivers to transmit information from a base station transmitter to the receivers. This information has been transmitted using a number of well known paging coding schemes and message formats, such as POCSAG or Golay coding schemes. Over the past few years, the predominant code transmission schemes used to signal paging receiver devices have changed from sequential tone base systems to formats based on multidigit binary code words, and the services offered to the user have evolved from simple alert only and alert plus voice signalling to more complex multifunction alerting with visual readout of numeric and alphanumeric data. Modern paging systems and paging receivers in particular have achieved multifunction capability through the use of microcomputers which allow the paging receiver to respond to information having various combinations of tone, tone and voice, or data messages. Coupled with the increased use of microcomputers in paging receivers has been the ability to use semiconductor memory to increase the storage capability of the paging receivers to recall data messages at a later time.
In the operation of such paging receivers, important factors involved in their operation have been the portability of the receiver, the size of the paging receiver, the cost of manufacturing the paging receiver, the limited energy available for the paging receiver, the limited availability of the radio spectrum, the fast response time required in today's activity, and the number of paging receivers included in the paging system. In such paging receivers, in order that the drain on the battery be minimized and to increase battery life, the paging receiver is systematically turned off and turned on to maximize the length of time energy is available from the battery. This is typically known as battery saving. Increasing the battery life is highly desirable for commercial marketability, thus operating the paging receiver in a limited energy environment becomes a necessity. The limited energy in which the paging receiver must operate constrains the type of electronic circuitry available for such paging receivers.
In addition, the paging systems that support the operation of paging devices have grown from small on-site in one city systems to very large wide-area systems that can cover most of the state or even an entire nation as evidenced by nationwide paging systems used in Europe. Further, the coding systems used in binary base signalling systems have evolved very rapidly as shown by the recent proposed plans to increase the bit rate for the standard POCSAG code from the transmission rate of 512 bits per second to 1200 bits per second.
The rapid pace of technology change in the paging industry has made the efficient operation of paging systems a very difficult task. A paging system operator often discovers that the paging receivers that were purchased last year do not have all the features provided by units purchased this year, or worse yet, the two sets of paging receivers require two different, and sometimes, incompatible signalling systems. As a result, many paging systems use a mixture of paging receivers and signalling schemes, and the system transmits a time multiplex signalling pattern that might consist of a lengthy sequence of six tone sequential paging codes, followed by a lengthy sequence of POCSAG tone only binary paging codes, followed by a sequence of numeric display signals transmitted in accordance with the Motorola Golay sequential code signalling format.
This mixture of code signalling formats, receiver device models, and device features causes operational problems in terms of system capacity, user queue time, and system flexibility as well as increased cost and problems associated with the repairing and maintaining a large number of different paging receiver models. It has also made it very difficult to add new features and enhancements to the system because of the incompatibility with the older units which cannot be discarded or obsoleted because of the initial capital investment.
A solution to this problem is to provide reprogramming for the paging receiver to allow the reconfiguration of the paging receiver to respond to different signalling formats and to provide greater feature capabilities of the paging receiver. The reconfiguring of the paging receiver falls into two types. First is factory reconfiguration in which the paging receiver is reprogrammed at the factory site or local service site. However, this type of reconfiguring ceases once the paging receiver leaves the factory or service area. A very simple example of factory programming is shown in U.S. Pat. No. 4,422,071 by DeGraaf. DeGraaf illustrates how to program the address code at the factory site.
The second type of reconfiguring is known as over-the-air programming. This type of reconfiguring is beneficial after the paging receiver is acquired by the user. The over-the-air programming of a paging receiver has been described in U.S. patent application Ser. No. 07/116,948 filed 06/11/87, having inventors Davis et al., and assigned to the assignee of the present application. The application describes a paging receiver in which the receiver can be modified by over-the-air commands. In particular, the paging receiver can be totally reconfigured via over-the-air commands and data transmission in terms of the signalling system it can decode and the features and options it can provide. The new selective call paging receiver is flexible enough to provide for a total change of the signalling system and correspondingly the operating characteristics of all the receivers operating within the system as well as providing the means for changing the features and options in any subset of the selectable paging receiver units.
In manufacturing these paging receivers, it is highly desirable to program the address and operating options into a nonvolatile memory offering in-system erase and reprogram capabilities. An example of such a reprogrammable memory is an Electrically Erasable PROM (EEPROM). An EEPROM allows the memory to be electrically erased and reprogrammed. This combination is well suited to a paging receiver operating environment.
In a paging receiver, the address and/or the options can then be modified without replacing the memory unit as was required with prior "one-time" programmable-address code plugs. From the manufacturing standpoint, it is highly desirable to provide for address code and options reprogrammability and over-the-air programming with one EEPROM unit. However, Applicants have discovered that this could allow for the inadvertent modification of data that was not meant to be modified included in the EEPROM. The inadvertent modification of data in the EEPROM was caused either by the reception of error in the signals or by the loss of program control in the microcomputer decoder typically used in these complex paging receivers. Therefore, it has become highly desirable to provide a single EEPROM unit that provides reprogrammable address and options and over-the-air programming in selected regions of the memory, but protects data programmed into a protected region of the EEPROM from inadvertent modification.